(a) Technical Field
The present disclosure relates generally to an apparatus and method for controlling an auto cruise speed. More particularly, the present disclosure relates to an apparatus and method for controlling an auto cruise speed, which can improve the auto cruise control performance in a section where a target vehicle speed changes.
(b) Background Art
Generally, an auto cruise control system of a vehicle is an apparatus for maintaining a constant speed or controlling an automatic speed while driving. When a driver sets a target vehicle speed (e.g., with a switch operation, such as speed setting button), the auto cruise control system controls the speed of the vehicle at the set target vehicle speed, significantly reducing the accelerator pedal operation of a driver and thus improving the driving convenience.
In this auto cruise control system, when a speed controller generates a speed control torque (i.e., auto cruise torque) command according to the target vehicle speed, through cooperative control between controllers, internal combustion engine vehicles (e.g., gasoline- or diesel-fueled vehicles) perform engine torque control including fuel supply control based on the speed control torque, enabling auto cruise driving that maintains the target vehicle speed. Also, for the auto cruise control, electric vehicles driven using a motor control the motor torque based on the speed control torque, and hybrid vehicles driven using a motor and an engine distribute power to the motor and engine so as to generate the speed control torque.
Commonly, the vehicle torque control for the maintenance of the target vehicle speed is performed through feedback and feedforward control. When the torque control of a power unit (i.e., engine and/or motor) is performed for the auto cruise driving, a required torque for correcting an error according to the target speed (i.e., a difference between the current speed and the target speed of a vehicle) is generated in the feedback control through Proportional Integral (PI) (or Proportional Integral Derivative (PID)) control. The required torque is determined within an available torque range of the power unit through the PI (or PID) control in accordance with the difference between the current speed and the target speed of a vehicle.
Also, a torque corresponding to a drive resistance of a vehicle is converted in the feedforward control, and the required torque determined in the feedback control is pre-corrected. Thus, final auto cruise torques (e.g., speed control torque and final wheel required torque) are determined. More specifically, the speed control during the auto cruise driving is performed by controlling the torque output of the power unit based on the auto cruise torque (i.e., torque command) that is the sum of the feedforward torque and the feedback torque.
At this point, in the feedforward control process for calculating the feedforward torque, the target speed is inputted to determine the drive resistance from the target speed, and a load torque required in accordance with the drive resistance is calculated. Also, in the feedback control process for calculating the feedback torque, PI (or PID) control is applied to follow up the target speed of a vehicle.
Hereinafter, typical auto cruise control processes will be described with reference to FIGS. 1 and 2.
As described above, the auto cruise torque, i.e., the speed control torque for controlling the output of the power unit during the auto cruise speed control, is determined by 1) the feedback control for correcting the speed error of the target speed and the current speed of a vehicle and 2) the feedforward control for correcting the drive resistance of a vehicle. For this, a speed controller 1a of an auto cruise control system includes a feedback controller 10 determining a torque for correcting a speed error by inputting the speed error of the target speed and the current speed of a vehicle, and a feedforward controller 20 determining a torque for correcting a drive resistance by inputting the target speed.
The drive resistance, as shown in FIG. 1, may include an air resistance and a rolling resistance. Also, in case of a vehicle equipped with a longitudinal acceleration sensor, the drive resistance, as shown in FIG. 2, may further include a slope resistance (i.e., gradient resistance) according to the road inclination angle (i.e., road slope). The rolling resistance and the air resistance are pre-calculated values in consideration of the hardware characteristics of the vehicle, and are determined in accordance with the vehicle speed. The slope resistance is determined in accordance with the road inclination angle calculated using a signal of the longitudinal acceleration sensor.
In this case, the longitudinal acceleration of a vehicle, which is a value sensed by the longitudinal acceleration sensor, is used for obtaining the road inclination angle θ together with the vehicle acceleration calculated from the vehicle speed. The road inclination angle may be expressed as the Equation below.
  θ  =            1      g        ⁢          (                        longitudinal          ⁢                                          ⁢          acceleration                -                  vehicle          ⁢                                          ⁢          acceleration                    )      
Here, g denotes a gravitational acceleration.
The torque for correction of the drive resistance, which is determined by the feedforward controller 20, is a torque that allows a net torque that is applied to the wheel shaft of a vehicle by the drive resistance to become 0, and is determined as a torque corresponding to the drive resistance (i.e., rolling resistance+air resistance+slope resistance) of a vehicle.
Finally, as shown in FIGS. 1 and 2, when the feedforward torque according to the drive resistance of a vehicle is determined in the feedforward controller 20, and the feedback torque is determined through the PI (or PID) control for removing the error between the target speed and the current speed of a vehicle in the feedback controller 10 (i.e., PI (or PID) controller), the auto cruise torque (i.e., wheel required torque) for maintaining the vehicle speed at the target speed is finally determined as the sum of the feedforward torque and the feedback torque, which are determined as above.
Meanwhile, the conventional auto cruise control system has the following limitations. For instance, the speed controller is designed based on the follow-up characteristics with respect to a constant target speed. Accordingly, in a situation where the target speed upwardly varies (in a form of lamp, for example) in accordance with the operation of auto cruise control functions such as “Acceleration”, “Deceleration”, “Tap-up”, “Tap-down”, and “Resume”, the control response is reduced, and the speed control performance of auto cruise is reduced. More specifically, in order to increase the target speed, a driver needs to operate a speed setting button such as “Acceleration” or “Tap-up”. Here, the operation “Acceleration” means that the target speed continuously increases while a driver is pushing the button, while the operation “Tap-up” means that the target speed increases by stages as much as a predetermined basic unit speed (e.g., 1 km/h) whenever the button is shortly pushed. When a driver intends to significantly increase the target speed, a driver may perform the “Acceleration” function closely to a desired target speed, and then may perform the “Tap-up” function to accurately set the target speed.
On the other hand, in order to decrease the target speed, a driver needs to operate a speed setting button such as “deceleration” or “Tap-down”. Here, the operation “deceleration” means that the target speed continuously decreases while a driver is pushing the button, while the operation “Tap-down” means that the target speed decreases by stages as much as the predetermined basic unit speed (e.g., 1 km/h) whenever the button is shortly pushed. When a driver intends to significantly decrease the target speed, a driver may perform the “deceleration” function closely to a desired target speed, and then may perform the “Tap-down” function to accurately set the target speed.
Also, the “Resume” function means that the target speed returns to a previous target speed which was used for the speed control.
FIG. 3 is a view illustrating limitations of the related art. When the target speed increases or decreases (e.g., in a form of lamp) through the foregoing operation in the auto cruise control system, speed drop and overshoot may occur as shown in FIG. 3 while the speed error is being corrected during the feedback control process.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure, and therefore, it may contain information that does not form the related art that is already known in this country to a person of ordinary skill in the art.